Transportation fuels, particularly motor gasoline, contain a relatively high level of aromatic components, such as benzene. These fuels, while relatively high in octane number, are facing ever growing difficultly in meeting environmental regulations with regard to emissions. This is primarily because of their high level of aromatics. Consequently, much work is being done to develop what has become known as "low emissions fuels". An important aspect of this work involves the substitution of non-aromatic components, having a relatively high octane value, for aromatic components of the fuel.
A class of non-aromatic components having relatively high octane value, which has been proposed for the production of low emissions fuels, is oxygenates. Non-limiting examples of preferred oxygenates for fuels include the unsymmetrical dialkyl ethers, particularly methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amylmethyl ether (TAME). Conventional methods for the manufacture of MTBE include the reaction of iso-butylene with methanol over cation-exchanged resins. This has created a significant demand for iso-butylene. Furthermore, there is also a demand in the chemical industry for other low carbon number olefins.
Low carbon number olefins, for example those having 2 to 10 carbon atoms, are typically obtained by the dehydrogenation of the corresponding paraffinic hydrocarbon. One method for light paraffin dehydrogenation is the so-called oxidative dehydrogenation process wherein light alkanes are reacted with oxygen over a mixed metal oxide catalyst to produce a mixture of olefin, water, CO.sub.x, and unreacted paraffin. While high conversions combined with high olefin selectivities can be achieved, this process has a number of disadvantages. One disadvantage is the loss of fuel value because of water and CO.sub.x formation. Another disadvantage concerns the relatively high costs of running the process. There are also problems concerning hazards associated with exothermic combustion reactions.
A more direct and preferred approach for producing low carbon number olefins, is direct dehydrogenation over a suitable catalyst to produce olefins and molecular hydrogen. This chemistry has recently received considerable interest, although high reaction temperatures in the range of 500.degree. C. to 650.degree. C. are required to obtain a significant equilibrium yield (e.g., 15-65%) of olefin. Moreover, under these reaction conditions light alkane hydrogenolysis to methane and ethane is a competing undesirable reaction. Most catalysts studied to date have not shown suitable selectivities for dehydrogenation versus hydrogenolysis. They have also suffered from rapid deactivation, necessitating frequent regeneration. As a consequence, the process economics have not been clearly favorable. Large incentives exist for catalysts which show relatively high selectivity for olefins and which have improved resistance to deactivation. It is also desirable that the catalyst be capable of being regenerated using relatively inexpensive procedures, such as treatment with air.
It was found by the inventors hereof that a carbonaceous catalyst will effectively catalyze the dehydrogenation of light alkanes. This is the subject of U.S. patent application Ser. No. 07/900,977, filed Jun. 18, 1992, which is incorporated herein by reference.
One source of carbonaceous material in some modern complex petroleum refineries is in fluid coking process units. In conventional fluid coking, in a process unit comprised of a coking reactor and a heater, or burner, a petroleum feedstock is injected into the reactor in a coking zone comprised of a fluidized bed of hot, fine, coke particles and is distributed uniformly over the surfaces of the coke particles where it is cracked to vapors and coke. The vapors pass through a cyclone which removes most of the entrained coke particles. The vapor is then discharged into a scrubbing zone where the remaining coke particles are removed and the products cooled to condense the heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 wt. % coke particles, is recycled to extinction to the coking zone.
The coke particles in the coking zone flow downwardly to a stripping zone at the base of the reactor vessel where steam removes interstitial product vapors from, or between, the coke particles, and some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser which moves them to a burner, or heating zone where sufficient air is injected for burning at least a portion of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking zone where the unburned hot coke is recycled. Net coke, above that consumed in the burner, is withdrawn as product coke.
Another type of fluid coking employs three vessels: a coking reactor, a heater, and a gasifier. Coke produced in the coking reactor is withdrawn and is passed to the heater where a portion of the volatile matter is removed. The coke is then passed to the gasifier where it reacts, at elevated temperatures, with air and steam to form a mixture of carbon monoxide, carbon dioxide, methane, hydrogen, nitrogen, water vapor, and hydrogen sulfide. The gas produced in the gasifier is passed to the heater to provide part of the reactor heat requirement. The remainder of the heat is supplied by circulating coke between the gasifier and the heater. Coke is also recycled from the heater to the coking reactor to supply the heat requirements of the reactor.
There is a need in the art for producing olefins in a more cost efficient manner, especially if a cheap source of catalyst, such as coke from a fluid coking unit could be used.